Analgesic drugs are roughly classified into a narcotic type and a non-narcotic type. The latter type of analgesics have been widely employed for clinical use due to their safety and easy manipulation.
In terms of clinical classification, there are two types of pains, i.e., acute pains and chronic or continuous pains. Acute pains are induced by specific causes such as wound and inflammation. Conventional analgesics have all been developed and clinically used for acute pains. On the other hand, chronic pains involve continuous pains over more than 6 months to a few years even after wounds causing acute pains have been healed. For such chronic pains, conventional analgesics are not always effective. A variety of drugs such as antidepressants and antispasmodics are applied to relieve chronic pains but there has been no decisive drug so far [Hiroshi Takagi, "Kyuseitsu-oyobi-Manseitsuno-Yakuri (Pharmacology of acute and chronic pains)", Nihon Ishikai Zasshi, Vol. 104, No. 1, pages 37-41, 1990].
Therefore, an object of the present invention is to provide an analgesic drug which is different from known analgesics in the action mechanism and is effective not only for acute pains but also for chronic pains.